The invention relates to the preparation of phosphorus and arsenic pentafluoride. Phosphorus pentafluoride can be reacted with lithium fluoride to form lithium hexafluorophosphate. Arsenic pentafluoride can also be used for the same purpose, and in the form of lithium hexafluoroarsenate can likewise be used as a conducting salt in lithium-ion batteries as disclosed in U.S. Pat. No. 5,427,874.
Phosphorus pentafluoride is prepared, for example, from phosphorus pentachloride and hydrogen fluoride. Arsenic pentafluoride is prepared from the metal and elemental fluorine. This method is disadvantageous in energy terms. Furthermore, elemental fluorine is very aggressive.
It is an aim of the present invention to provide a method for the preparation of pentafluorides of phosphorus and arsenic which is technically simple to perform.
This and other aims are achieved in accordance with the invention by providing a method of preparing an inorganic pentafluoride corresponding to the formula MF5, where M represents P or As, comprising reacting a trihalide corresponding to the formula MX3, where M has the meaning given above and X is fluorine, chlorine or bromine, with chlorine, bromine or iodine and with HF.
The preparation of MF5 in accordance with the invention, wherein M represents P or As, is effected by the reaction of MX3, wherein M has the meaning given above and X is fluorine, chlorine or bromine, with chlorine, bromine or iodine and with HF.
Preferably the method according to the invention is used for the preparation of phosphorus pentafluoride. The starting point preferably is phosphorus trichloride or phosphorus tribromide, or arsenic trichloride or arsenic tribromide, respectively. The respective starting compound is preferably reacted with chlorine or bromine, in particular chlorine, in order to oxidize the trivalent starting compound into the pentavalent state.
It has been found to be particularly advantageous if the method is performed continuously, although batchwise operation is, of course, possible. In the preferred, continuous procedure, it is possible to proceed in such a way that an excess of the trivalent starting compound is present in the reactor, relative to the oxidizing agent chlorine, bromine or iodine. Alternatively, an excess of the oxidizing agent can also be used. Good results are also achieved using stoichiometric equivalent amounts of the reactants. Hydrofluoric acid is advantageously used in at least the stoichiometrically required amount. HF may be used in an excess, and then also serves as a solvent. The reaction mixture may thereby comprise up to 70 wt.-% or more HF. Also, sufficient hydrogen fluoride should also constantly be present in the reactor, so that the product of the process (namely the pentafluoro compound) is evaporated off continuously in an amount corresponding to the addition of the starting compounds. For example, it is possible to proceed such that initially the trivalent starting compound and also sufficient hydrogen fluoride is provided and the oxidizing agent is introduced into this mixture. The reactants can then be introduced continuously into the reactor. By regulating the amount of the oxidizing agent or the trivalent compound which is added, it is possible to control the reaction rate. It is recommended to stir the reaction mixture.
Advantageously, the reaction is carried out at a temperature above xe2x88x92100xc2x0 C. Particularly good results are obtained in the temperature range from xe2x88x9225xc2x0 C. to 100xc2x0 C., in particular in the temperature range from 0xc2x0 C. to 40xc2x0 C. It is thereby possible, if desired, to operate under elevated (i.e. superatmospheric) pressure, or alternatively at standard (atmospheric) pressure, e.g. in autoclaves at autogenous pressure.
Due to the great sensitivity of the compounds to hydrolysis, it is advantageous to operate under an inert gas, and/or with moisture excluded.
The advantage of the method according to the invention is that the reaction can be performed easily, since the starting compounds can be metered readily, and the reaction can be simply controlled by appropriate metering of the oxidizing agent or the trivalent starting compound. The method is particularly well-suited to the preparation of phosphorus pentafluoride from phosphorus trichloride, bromine or chlorine, and HF.
The following examples are intended to illustrate the invention in further detail without restricting its scope.